The Navigation Satellite Timing and Ranging (NAVSTAR) GPS is a space-based satellite radio navigation system developed by the U.S. Department of Defense (DoD). GPS receivers provide land, marine, and airborne users with continuous three-dimensional position, velocity, and time data.
The GPS system consists of three major segments: Space, Control, and User as illustrated in FIG. 1. The space segment consists of a nominal constellation of 24 operational satellites which have been placed in 6 orbital planes above the Earth's surface. The satellites are in circular orbits in an orientation which normally provides a GPS user with a minimum of five satellites in view from any point on Earth at any one time.
Each satellite continuously broadcasts navigation data. This navigation data, which is computed and controlled by the GPS Control Segment, includes the satellite's time, its clock correction and ephemeris parameters, almanacs, and health status for all GPS satellites. From this information, the user computes the satellite's precise position and clock offset.
The controlling segment consists of a Master Control Station and a number of monitor stations at various locations around the world. Each monitor station tracks all the GPS satellites in view and passes the signal measurement data back to the Master Control Station. There, computations are performed to determine precise satellite ephemeris and satellite clock errors. The Master Control Station generates the upload of user navigation data from each satellite. This data is subsequently rebroadcast by the satellite as part of its navigation data message.
The user segment is the collection of all GPS receivers and their application support equipment such as antennas and processors. This equipment allows users to receive, decode, and process the information necessary to obtain accurate position, velocity, and timing measurements.
GPS based position solutions are inherently poor under low vehicle dynamics. Some current systems use a hard wired speed signal or odometer input to help correct or calibrate distance errors under low or zero speed conditions. This hard wired approach increases the overall system cost, requires specialized knowledge of the vehicle's wiring system, and requires a specialized wiring harness to interface to each individual automobile. In addition, certain systems require a calibration procedure to determine the zero offset of the motion sensor. For example, in a system using a compass or gyro, the user must press a calibration button and drive in a circle and come to a stop for a minimum period of time to determine zero offsets for those devices. Using the road network of a map database, if a vehicle is travelling on a straight path, the system can calculate the zero offset for the gyro. Odometers were previously used for zero motion but are generally ambiguous below 3 mph and also required hard wiring. Other systems require a rotational sensor to determine a vehicle's change in heading under these low dynamic or other unfavorable GPS conditions.
Accordingly, there is a need for an improved vehicle navigation system which can more accurately, efficiently and cost-effectively reduce errors in the position determination at low or no speeds than current vehicle navigation systems.